Today radio telecommunications networks are common and user equipment connected to a base station within the radio telecommunications network transmits data to the base station using an uplink radio carrier and receives data from a base station over a downlink radio carrier. In radio telecommunications networks a scheduler schedules downlink and uplink transmissions. For example, considering the 3GPP Long Term Evolution (LTE) air interface, a scheduler is situated in an evolved NodeB (eNB) and schedules both the downlink and uplink transmissions between the eNB and user equipments (UEs) in the system.
For the transmission to be successful, the eNB and the UEs have to agree on a wave form to be used. Therefore, scheduling decisions of the scheduler, such as downlink assignments and uplink grants, are transmitted from the eNB to each UE on a Physical Downlink Control Channel (PDCCH) of the eNB.
For PDCCH only quadrature phase shift keying (QPSK) modulation is used, and coded bits of the QPSK are mapped to the resource elements in the first three Orthogonal Frequency Division Multiplexing (OFDM) symbols of a subframe. This is done in the following way.
A resource element is a complex symbol corresponding to one subcarrier in one OFDM symbol. Resource elements are arranged into Control Channel Elements (CCEs), each containing 36 resource elements. Since the PDCCH uses QPSK modulation, this corresponds to 72 (2 times 36) physical bits per CCE. The number of CCEs that are available in each subframe depends on the bandwidth and common channel configuration of the cells.
A downlink assignment or uplink grant is coded as downlink control information (DCI) and these contain in the order of 20 to 70 information bits each. After channel coding, the coded bits are mapped to 1, 2, 4 or 8 CCEs, depending on the channel quality of the link between the eNB and the UE. A Radio Network Temporary Identifier (RNTI) of the UE indicates which group/s of CCEs that are allowed for this UE, and the UE blindly decodes all allowed combinations.
As described above, a control message, such as a DCI, is channel coded and the coded bits are mapped to a group of L CCEs, where L=1, 2, 4 or 8; L is called the aggregation level. For each value of the aggregation level, each UE has a UE specific search space of allowed CCEs that can be used to send the control message. The UE then blindly decodes all possible combinations of aggregation levels and allowed CCEs.
The performance of a physical channel like the Physical Downlink Control Channel (PDCCH) can be measured in Block Error Rate (BLER), i.e. the probability that a transmission of the PDCCH fails. The PDCCH should have a BLER below a certain value for the UE to be considered in good connection with the eNB. Note that if the information sent of the PDCCH is lost, the traffic on the traffic channels, Physical Downlink Shared CHannel (PDSCH) and the Physical Uplink Shared CHannel (PUSCH), can not be sent, since the UE is not informed on the waveform of the transmissions, and therefore the whole transmission attempt is lost.
The BLER depends on the modulation, code rate used and on the effective Signal to Interference and Noise Ratio (SINR) at the receiver. The BLER decreases with decreasing code rate and with increasing SINR. Looking at BLER as a function of SINR for different code rates one may discover that the code rate of the PDCCH is inversely proportional to the number of CCEs used.
A Link Adaptation for PDCCH calculates the effective SINR over the bandwidth and then uses the information contained in a graph. The number of CCEs is chosen so that, for the calculated effective SINR, the expected BLER is below the target BLER for PDCCH.
Since only the PDCCH uses QPSK modulation and not more than 8 CCEs (576 coded bits) are available to transmit each DCI, the code rate is limited from below, and a minimum SINR is required for successful transmission of the PDCCH.
A minimum SINR implies limitations both on tolerated path loss, i.e. on coverage, and on the tolerated interference level, i.e. on the overall load in the system. This implies a problem in that the control channel, such as the PDCCH, may be limiting both on coverage and capacity, that is, the performance, of the radio telecommunications technology in certain scenarios. For example, if a failure of a transmission is eventually detected, a new attempt to perform the transmission is made; this will take time, delaying the transmission, resulting in a reduced performance of the radio telecommunications network.